A digital-to-analog converter (DAC) is a device for converting a digital code to an analog signal. For example, a DAC can convert an 8-bit digital signal into an output voltage or current having an amplitude representing the digital code. Two common examples of DACs are the “R-string” DAC and the “R-2R ladder” DAC. Another example is the parallel resistors architecture (PRA) DAC. Advantages of the PRA-DAC over the “R-string” DAC and the “R-2R ladder” DAC include that the PRA-DAC has a constant output impedance and inherent monotonicity compared to “R-2R ladder” DACs.
When an input (e.g., a digital code) is changed, the output (e.g., an analog signal) of a DAC settles to a value after a delay called a settling time. The settling time depends on the output resistance Rout of the DAC and a capacitive load CL at the output of the DAC. In particular, the settling time depends on a time constant that can be defined by the product of Rout and CL. The settling time can limit a conversion speed of the DAC.